In the past, numerous different methods have been used for singulating or dicing a semiconductor wafer, the process of dividing a semiconductor wafer into individual devices. The two most widely used methods at this time are sawing using a diamond saw blade and laser scribing, using a focused laser beam to cut through the wafer. Neither method is ideal. Both result in a significant loss of material during the cutting process. As the size of semiconductor devices get smaller, the width of the line of lost material during the scribing process becomes comparable to the width of the device. If the width of the line of material lost during the scribing process could be made significantly smaller, many more devices could be made on each wafer, resulting in a large savings in the cost of fabricating the devices. In addition, both sawing and laser scribing cause damage along the cut edges of the devices that can result in rejected devices during visual inspection and in some cases cracking that can cause device failure in the field.
Since the invention of plasma and reactive ion etching in the 1970s, many have proposed using these processes for wafer singulation. These processes potentially could decrease the material loss during the dicing process by etching very narrow scribe lines through the semiconductor wafer. In addition, since the etch process takes place at a microscopic level and involves no heat or mechanical grinding, the edges of the semiconductor devices are not damaged by the process. In order for a plasma etching or a reactive ion etching process to be effective in wafer dicing, it would have to etch very deep, narrow trenches in the scribe streets of the semiconductor wafer and it would have to etch at a very fast etch rate to be economically attractive. These two conditions have been achieved in the last several years by employing the teachings of Teixeira, et al. (U.S. Pat. No. 6,417,013) building on the work of Laermer, et al. (U.S. Pat. No. 5,501,893). An issue that remains to be resolved is a cost effective method of removing the back metal that remains in the scribe street after the etch process is completed.
Semiconductor wafers usually have one or more metal layers applied to the back of the wafer during fabrication to provide ohmic contact and/or ease of die attach during packaging of the devices. These layers of metal are not readily etched using dry etch processes.